The present invention relates to a ceramic gas-turbine nozzle with fine cooling holes and a method for preparing the same. The ceramic gas-turbine nozzle with fine cooling holes of the present invention can be preferably used as, for example, a dynamo gas turbine nozzle.
Recently, dynamo gas turbines have been intensively developed in order to achieve energy-savings; decreased environmental pollution and usability of various fuels. Particularly, the research for development of a the gas turbine with large heat efficiency or high energy savings is vigorously performed, and causes the related researchers to focus their attention on a problem that the inlet gas temperature (TIT) of the turbine is elevated to improve the efficiency of heat engines. In fact, a metal gas turbine using a heat-resistant alloy has a maximum inlet gas temperature of 1350.degree. C.
An attempt has been made to further elevate the turbine inlet gas temperature by applying ceramics which are superior excellent in heat resistance than the heat-resistant alloy to parts such as gas turbine blades, and so gas turbines using a ceramic have already been developed which have almost the same TIT as those using a heat-resistant alloy. Thus, the current development of gas turbines aims at preparing those with a TIT of nearly 1500.degree. C.
When a ceramic component, however, is used for a gas turbine nozzle having a TIT of more than 1500.degree. C., a local area having a TIT of more than 1600.degree. C. may be present. Under such circumstances, even if a desired heat-resistant ceramic is obtained, there arise problems of decrease in mechanical strength, or potential influence of erosion or corrosion, reduced reliability, shortened service life and the like.
The present inventors have now developed a ceramic component with fine cooling holes in which a refrigerant carrier flows through the fine holes bored at predetermined positions in the ceramic component to improve the heat resistance of the ceramic component; Japanese Patent Laid-Open No. 4-219205 (Japanese Patent Application No. 3-87581). Since the surface of this ceramic component is cooled by a refrigerant carrier, even if the TIT is elevated to 1500.degree. C. or more, the surface of the ceramic gas-turbine nozzle is kept at lower temperatures, e.g., about 1100.degree. C.; thus there are no problems about decreased reliability and mechanical strength and the like under a higher temperature. The lower temperatures hardly have adverse effects on the heat efficiency of these gas turbines.
However, the publicly known methods have disadvantages if the ceramic component is provided with the fine cooling holes. Examples of the conventional methods by which the fine holes are bored include a method where piano wire or the like is placed in the mold at the time of injection molding, or grinding processing or supersonic processing after sintering.
For example, when injection molding is performed, there are problems that the piano wire is cut off because of the increased pressure of injection molding, and the position and the like of fine holes are limited depending on the mold structure. There are also problems that the fine holes having smaller diameters may make it impossible to successfully release the fine hole portions from the mold.
Similarly, only straight fine holes can be made by the grinding process or supersonic process after sintering, with the limitation of their deepness. These processings make the beveling processing impossible. Particularly, when the fine hole is to be made so as to have a diameter of about 0.5 mm, its processing time will be greatly prolonged and its mass production will be poorly attained.
Alternatively, when a ceramic is applied to the gas-turbine nozzle parts, there arises a problem characteristic of the gas turbine nozzle. Having large size, the gas turbine nozzle provides complex shapes, so that the conventional methods for preparing ceramic parts have great limitations of parts' shapes and dimensions, which are ascribed to their molding shape, moldability, sinterability, processability and the like. For example, in the case of a large-sized gas-turbine nozzle, the larger body makes it difficult to be molded by even any method such as injection molding and press molding. Particularly, the present method of injection molding does not provide the resulting moldings with uniform density, and so their large deformation caused after sintering cannot mold the compacts or parts with close shape-precision.
Examples of gas turbines having more complex shapes include a gas turbine in which plural blades are put between the inside shroud portion and the outside shroud portion to align linearly or in the form of an arc. There arises a great problem that it is difficult to mold a gas turbine nozzle having a complex shape, which causes undercuts. Thus, when forming a gas turbine in such a shape which will cause undercuts, it is clearly impossible to prepare compacts using the ordinary mold.